The present invention relates to digital communication systems and more particularly to digital communication systems that employ a medium access control (MAC) protocol to control access to a shared transmission medium.
There exists a great need for broadband access to the Internet and other forms of broadband communication between remote points. Perhaps the ideal medium for broadband communication is optical fiber. Optical fiber has, however, not yet been extended to all, or even a majority of, the businesses and residences that could benefit from broadband services. Other technologies seek to leverage existing infrastructure to deliver broadband connectivity. For example, DSL technology provides broadband services over the copper twisted pair wire that makes up the bulk of local telephone service infrastructure. The applicability of DSL is however limited by distance from the switching office. As this distance increases, available bandwidth decreases until the service becomes entirely unavailable.
Cable systems may also be employed to provide two-way broadband digital communications. Cable systems have been constructed for broadcast television and thus employ a shared transmission medium, coaxial cable, to distribute analog television signals from a head end to a great number of subscribers. In a conventional cable television system, the subscribers only receive and do not transmit. To make use of the cable infrastructure for two-way digital services, one must provide a MAC protocol to prevent subscribers from interfering with one another when transmitting upstream to the head end. One such protocol that has been developed is referred to as the MCNS protocol. The MCNS protocol is described in Data-Over-Cable Service Interface Specifications, Radio Frequency Interface Specification, SP-RFI-I04-980724, (Cable Television Laboratories, 1997), the contents of which are herein incorporated by reference.
Cable systems also do not represent a universal solution to the problem of broadband access. The available transmission medium is physically accessible to many users. If too many users are competing for access to the medium, broadband service may be unavailable to some or all of those requesting it. The limitation of medium availability is particularly important where it is desirable to service businesses that are providing high bandwidth web content to many customers. Also, providing any kind of digital two-way service may require substantial and expensive upgrades to the cable plant to allow for the necessary quality of transmission.
Wireless technology does not require new deployment of wire, cable, or optical fiber and does not depend on the characteristics of existing infrastructure. Wireless is therefore seen as a desirable complement to the other techniques for providing broadband internet access to businesses and residences.
Wireless systems are also, however, constrained by the capacity of a transmission medium common to many users. Typically, on a given frequency, at any one time, only one subscriber or the head end may transmit. Just as with the two-way broadband cable systems, there must be a MAC protocol to control access to the shared medium.
It would be desirable to simply adopt a protocol already developed for cable applications to the wireless context. A cable MAC layer like MCNS is already implemented in low cost chip sets. The operational characteristics of MCNS are well known. Hardware and software to implement higher layer operations has already been designed to interact with MCNS. Furthermore, it is desirable to maintain parts commonality between wireless modems and cable modems to the extent possible.
Unfortunately, the characteristics of the wireless physical layer make it difficult to apply the cable MAC layer protocols without significant modifications that undermine the advantages of using a common protocol for both cable and wireless. MCNS, for example, provides that subscriber units must periodically send data to the head end so that the head end may make a received power measurement. A wireless system requires more frequent power adjustments than a cable system because of the inherent variation over time in the wireless channel response. MCNS, however, requires relatively high processing overhead to make a power measurement at the head end and adjust subscriber unit output power in response. Due to this high processing overhead, it is very difficult to control power in a wireless system controlled by a cable MAC layer protocol such as MCNS.
Systems and methods for employing digital two-way cable MAC protocols for multi-user wireless communication systems are provided by virtue of the present invention. In one embodiment, a physical layer TDMA framing structure is provided to complement the MAC layer framing. A single MAC layer frame is divided into multiple physical layer frames. The physical layer frames are divided among frames dedicated to transporting data between MAC layer entities and frames dedicated to transporting physical layer control information. Constant delay in transmission through the physical layer is preserved.
A first aspect of the present invention may be applied within a communication system employing a MAC layer. A first node of the communication system includes: a MAC layer processor, and a physical layer control processor that divides a MAC layer transmission frame into a plurality of physical layer frames, and that allocates at least a first one of the physical layer frames to transmission of data originating with the MAC layer processor. The physical layer control processor reserves a second one of the physical layer frames for transmission of physical layer control information by either the first node or another node of the communication system.
A second aspect of the present invention may be applied in a communication system employing a medium access control (MAC) layer. A central access point includes a MAC layer processor that allocates a MAC layer transmission frame to a first subscriber unit, and a physical layer control processor that allocates a first physical layer transmission frame within the MAC layer transmission frame for transmission of MAC layer data by the first subscriber unit, and that allocates a second physical layer transmission frame within the MAC layer transmission frame for transmission of physical layer control information by the first subscriber unit or a second subscriber unit.
A further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings.